High voltage electro-plating method



6, 1955 s. c. ROCKAFELLOW 2,726,203

HIGH VOLTAGE ELECTRO-PLATING METHOD Filed June 6, 1955 5 Sheets-Sheet 1 NCATHODE 7 ANODE AVERAGE INVENTOR. 670 427 6 EOCAAFEAlOW J ATTORNEY 1955 s. c. ROCKAFELLOW 2,726,203

HIGH VOLTAGE ELECTRO-PLATING METHOD Filed June 6, 1955 3 Sheets-Sheet 2 AC. SUPPLY 48 4a 4a 4a AVERAQLCW Jig. 4

INVEN TOR. drawer C. FacKw-zu an/ ATTORNEY Dec. 6, 1955 s. c. ROCKAFELLOW 2,726,203

HIGH VOLTAGE ELECTRO-PLATING METHOD Filed June 6, 1955 3' Sheets-Sheet 3 ANODE CATHODE INVENTOR. 670 427 foomrsuow ATTORNEY United States Patent HIGH VOLTAGE ELECTRO-PLATING METHOD Stuart C. Rockafellow, Plymouth, Mich., assignor to Robotron Corporation, Detroit, Mich., a corporation of Michigan q Application June 6, 1955, Serial No. 513,250

8 Claims. (Cl. 204-50) This invention relates to a method for the application of electrical power from an alternating current source and particularly it relates to the application of uni-directional power from an alternating current source for electro-plating purposes, wherein said pulses are each of less than one half cycle in duration.

This application is a continuation-in-part of my application Serial No. 302,669, filed August 5, 1952, entitled High Voltage Electro-plating-Means, now abandoned.

In conventional electro-plating practice, it has been common to supply direct current of relatively high amperage and low voltage. For example, amperages commonly employed have varied from about amperes to about 250 amperes per square foot current density, depending upon the type of plating being done and particularly upon the metal being plated, and typical voltages employed varied from about 3 volts to about 12 volts. It has been discovered that the plating may be effected more quickly than in conventional processes by supplying the current to the plating electrodes by a series of uni-directional pulses wherein said pulses are each of less than one half cycle duration and, quite unexpectedly, it has also been discovered that plating so applied is actually substantially better than plating applied by conventional process in that the plating is more firmly and tightly knit to the base material than in plating applied by conventional processes. Further, it has been discovered that such pulses can be applied to the plating electrodes at sufliciently spaced intervals that, while the peaks of said pulses may be of the order of 350 volts and several thousand amperes per square foot current density, the average voltage and amperage will remain within convenient limits.

Thus, by the method of this invention, it is possible to effect better plating than can be obtained by the previous methods and to do so more rapidly, at less cost in power and at less cost in the size of equipment required for handling the current being utilized.

It is further familiar in plating practice that gases often collect on one of the electrodes and thereby materially diminish the freedom with which the plating current is enabled to flow. Therefore, if a method could be supplied by which this gas could be rapidly and efficiently removed substantially simultaneously with its formation, the plating electrodes would remain clean of it and the loss in efficiency now experienced would be eliminated. While this procedure broadly has previously been suggested, I have found it is possible to do this with particular eiiiciency as a refinement of the invention herein disclosed by permitting momentary pulses of negative current to be interspersed between positive pulses, said negative pulses likewise being of less than one half cycle duration. The positive pulses are of relatively large magnitude and constitute the plating current. Negative pulses are relatively small but will function to strip the gases from the cathode (or anode in the case of chlorine and certain similar gases) and thereby eliminate the undesirable polarization.

Accordingly, a principal object of the invention is to provide a method by which electro-plating may be carried out more efiiciently and economically than has hitherto been possible.

A further object of the invention is to provide a method by which electro-plating may be carried out more efficiently and quickly, and by which a stronger and tighter plating may be applied to a base metal, than has hitherto been possible.

A further object of the invention has been to provide a method by which electro-plating may be carried out with less power requirements than was characteristic of previous practice.

A further object of the invention has been to provide a method by which a smaller average amperage is used than was characteristic of previous practice.

A further object of the invention has been to provide a method by which an electro-plating process may be carried out from an alternating current source without the requirement of unduly complicated current control apparatus.

' Other objects and purposes of the invention will become apparent to persons acquainted with equipment of this general type upon a reading of the following disclosure and inspection of the accompanying drawings.

In the drawings:

Figure 1 represents a circuit diagram of a typical circuit by which the method of the invention may be practiced.

Figure 2 illustrates the voltage supply to the plating electrodes of the apparatus of Figure 1.

Figure 3 is a circuit diagram representing an alternate circuit by which the method of the invention may be practiced.

Figure 4 represents the voltage supply to the plating electrodes by the apparatus of Figure 3.

Figure 5 is a circuit diagram of a still further modified circuit for practicing the method of the invention.

Figure 6 represents the voltage supply to the plating electrodes by the apparatus of Figure 5.

IN GENERAL In general, my invention comprises rectifying, by either single wave or full wave rectification, an alternating potential and including in, or'with, said rectifying circuit means passing to the plating electrodes pulses comprising only a predetermined portion of the respective halfwaves of said alternating potential. Said pulses are spaced apart in time sufficiently that the average amperage and average voltage applied to said electrodes will both be as low as desired. For example, pulses of 350 volts will be spaced apart sufiiciently that the average voltage will be from 3 to 12 volts. With conventional electrolytes, this may supply current pulses of several'thousand amperes at their peaks and an average amperage much lower. This process has been found fully to accomplish the objects and purposes above set forth.

While it is apparent that a variety of circuits may be provided for carrying out the foregoing method, one preferred form of circuit comprises a single or full wave rectifier connected to a source of alternating potential and operating in conjunction with a switch and a transformer. Said switch is preferably of electronic nature and may be controlled by a phase shift circuit in such a manner that it commences conducting with respect to each wave form at a point after the beginning point of each respective wave. Preferably, however, it commences conduction somewhat following the high point' of a half-wave and continues to the end thereof. In order to secure the necessary sharpness in both the rise and fall of the pulse, the conduction will begin near the extreme end of the halfwave form, preferably within about the last thirty degrees thereof. Thus, the wave-form itself will provide the sharp fall desired and no special cut-off means are required.

DETAILED DESCRIPTION It is believed that the method of the invention may be best understood by reference to apparatus by which the method may be practiced. Accordingly, a detailed description of such apparatus will be set forth.

Figures 1 and 2 represent the simplest form of appara- Patented Dec. 6, 19 55 tus by which the invention may be practiced and will further illustrate the advantages of the method concerned. A plating tank of any conventional sort is indicated at l and said tank is provided with the electrodes 2 and 3. One of said electrodes, here the anode 3, is connected through the conductor 4 to one side of the A. C. current supply 6 and the other electrode, here the cathode 2, is connected by a conductor 7 through a switch, here a thyratron 8, and thence by the conductor 9 to the other side of said A. C. supply 6.

Said thyratron is of conventional sort and includes a cathode, an anode and a control grid.

Said control grid 11 is connected through the secondary winding 12 of a transformer 13 to the cathode of said thyratron.

A phase shift circuit 14, of any conventional type, is provided to etfect conduction of said thyratron at a desired point on the wave form of the A. C. supply. In the particular form used in this embodiment, a transformer 16 is connected by its primary winding 17 to the A. C. supply. Its secondary winding 18 is connected in series with a capacitor 19 and a potentiometer 21. The primary winding 22 of the transformer 13 is connected to the output terminals of said phase shift circuit, said output terminals being a terminal 23 located between said capacitor 19 and said potentiometer 21 and a terminal 24 center tapped into the secondary winding 18 of the transformer 16.

It will be recognized that when there is no shift in phase between the output of the transformer 13 and the A. C. supply, the grid and the cathode of the thyratron 18 will alternate at the same potential and conduction will be prevented. However, by shifting the phase of the potential applied to the grid 11, the thyratron may be caused to conduct a predetermined period of time following the beginning of each wave form. If such predetermined period is such that conduction occurs during the period indicated by the shaded area 26 in Figure 2, then there will be supplied a series of sharp pulses to the electrodes of the plating electrodes 2 and 3.

When making these pulses of sufliciently short duration, as from /2 to 2 milliseconds it is possible to have peak current densities of several thousand amperes per square foot of material being plated, as contrasted to current densities of the order of to 250 amperes per square foot of material being plated as is common in present conventional practice. This results in a substantial reduction in the overall time required for a given item of plating and, in addition, it has been found that even with this increased speed in plating the quality of the work is improved such that a finer grain structure is obtained as well as more tightly packed grains which effects a some what reduced porosity. Further, the adhesion between the plating material and the base material is materially increased.

Turning now to the apparatus shown in Figure 3 as illustrating further apparatus by which the method of the invention may be practiced, there is provided a full wave rectifier circuit for the supplying of the plating electrodes.

Much of this circuit is identical to that shown in Figure 1 and hence a very brief description will be sufficient to make it clearly understood. From a standard A. C. source 31, such as 240 volts, an alternating potential is supplied to the primary winding of a main transformer 32 and a control transformer 33. The transformer 32, through its secondary winding, supplies the plating electrodes 34 and 36 through a pair of opposed thyratrons 37 and 3B. The grids of said thyratrons are connected to their respective cathodes through the secondary windings 39 and 41 of a transformer 42 whose primary winding is connected to the output terminals 43 and 44 of'a phase shift circuit 46. This circuit provides full wave rcc tification, as indicated in Figure 5, and by adjustment of the potentiometer 47 the phase shift circuit may be caused to delay the conduction of the thyratron with respect to the wave form of the A. C. supply such that the plating electrodes are supplied with pulses during the periods indicated by the shaded areas 48 appearing in Figure 5. This will effect plating with the same advantages as above indicated for the form shown in Figures 1 and 2 and the work will proceed still more rapidly.

In Figure 5 there is illustrated a further modification by which a negative pulse may be applied to the plating electrodes intermediate the application thereto of the positive pulses above described. It will be recognized that much of the apparatus shown in Figure 5 is similar to that shown in Figure 1 and in the interest of brevity, the parts which are identical have been similarly numbered and description thereof will not be repeated. However, there is added to the circuit shown in Figure 1 a further thyratron 51 connected in parallel with the thyratron 8 but in opposite polarity and having its grid connected through the secondary winding 52 of a transformer 53 to the cathode of said thyratron 51.

A second phase shirt circuit 55 is connected to the secondary winding 18 of the transformer 16 and the primary winding of said transformer 53 is connected to its output terminals 54 and 56. It will be apparent, of course, that the secondary winding 52 might also be a second secondary winding of the transformer 13 but the two separate circuits as described will be preferable in some instances, all of which will appear clear below.

In the device here shown, positive pulses during the period represented by the shaded area 57 in Figure 6 will be transmitted by the thyratron 8 in the same manner as above described in connection with Figure l and the point of beginning of said pulses with respect to the wave form 58 will be determined by the setting of the potentiometer 21. However, in this circuit, the negative pulses will be transmitted by the thyratron 51 in pursuance of its control by the phase shift circuit 55 at a point with respect to the wave form 59 determined by setting of the potentiometer 61. The potentiometer 61 will be set to permit conduction through the thyratron 51 only at the extreme end of the wave form and thus its conduction will be during the period shown by the shaded area 62. This provides a very short but very sharp pulse of current across the plating electrodes in a reverse direction from the plating current. This pulse will be kept sufiiciently small that it will not interfere materially, if at all, with the plating operation as such but it will have suflicient effect to strip off gases which may accumulate on the electrodes and thereby it will keep said electrodes clean and at a maximum conductive evel. For example, if the positive pulses occupy the last 20 degrees of its half-wave form, the negative pulse may occupy the last 5 degrees of its half-Wave form.

Although thyratrons have been used throughout to illustrate circuits embodying principles of the invention, it will be recognized that a variety of other types of switch devices, and particularly including ignitrons, may be used in circuits which will be obvious in the light of the foregoing.

Specific examples follow:

' Reference Example 1 As a reference example a plating operation was carried out in the following manner. A standard copper solution" was prepared consisting of copper cyanide 3.5 ounces per gallon of water, sodium cyanide 4.5 ounces per gallon of water, Rochelle salt 4 ounces per gallon of water, and caustic soda /2 ounce per gallon of water. A copper bar was immersed in the solution and comprised the plating anode. A steel plate 3 inches by 5 inches by A1 inch was placed in the standard copper solution and comprised the cathode. A D. C. current was applied thereto at a voltage of 5 volts and with a current of 30 amperes per square foot. The plating operation was continued for seconds. Following the plating operation the thickness of the copper plating was noted,

the plated plate was exposed to a standard 30 day salt spray test and the grain structure was observed.

Reference Example 2 A further reference experiment was'carried out by first preparing a standard cadmium solution comprising cadmium oxide 3 ounces per gallon of water and sodium cyanide 18 ounces per gallon of water. A cadmium bar was immersed in the solution and comprised the plating anode. A 3 inch by 4 inch by inch steel plate was immersed therein and comprised the cathode. A constant D. C. current was applied thereto at a voltage of 8 volts and a current density of 20 amperes per square foot for a period of 11 minutes. The coating deposited was measured at 0.0003 inch and the density of cadmium deposited was observed.

Reference Example 3 The procedure of Reference Example 2 was followed using a hollow steel cylinder in place of the steel plate used in Reference Example 2, said cylinder being a standard one half inch steel pipe. The cylinder was held stationary with respect to the anode and upon removal from the plating operation the thickness of cadmium de- Using the standard copper solution as above described, a copper anode and a steel plate of the same character and size as above described, said plate was immersed in said solution as a cathode and subjected to a current applied according to the invention wherein the voltage was at 160 volts peak and the current density was about 170 amperes per square foot at the peak, the average voltage being about 6 volts and the averagecurrent density being about 19 amperes per square foot. The plating time was 15 seconds. Upon the removal of the plate from the plating solution it was found that the plating thickness was the same as in Reference Example 1, the adherence following the same 30 day salt spray test as above mentioned was better than in Reference Example 1 and the grain structure indicated was more densely.

packed than in Reference Example 1. When the conditions of Reference Example 1 were modified to secure the same plating thickness in 15 seconds, the density and adherence Was much inferior to that obtained here.

Example 2 Using a cadmium bar as an anode and a steel plate of the same size as used in the above reference examples as a cathode, the electrodes were placed in a standard cadmium solution as above described and plating current according to the method of the invention was applied at 160 volts peak, density of 400 amperes per square foot at the peak with an average voltage of about 6 volts and an average current of about 8 amperes per square foot. The plating was continued for 6 minutes. On removal of the plate from the solution it was found that the thickness of cadmium deposited was 0.0003 inch and that the cadmium density was about the same as with the plate referred to in Reference Example 2. When the conditions of Reference Example 2 were modified to secure the same plating thickness in 6 minutes, the density and adherence was much inferior to that obtained here.

Example 3 The procedure of Example 2 was repeated with a one-half inch steel pipe of exactly'the same size and character as that used in Reference Example 3. As in Reference Example 3, the steel cylinder. was held motionless with respect to the anode. Upon examination of the plated cylinder, it was found that the cadmium coating was not as thick on the side which faced the anode as it was in the case of Reference Example. 3 but that it extended further around the pipe and was of more uniform thickness throughout its extent around the pipe than in the case of Reference Example 3.

Experimental work determining the effect of varying peak voltages was carried vout in connection with the subject matter of my application Serial No. 513,210 entitled Method for Plating by Condenser Discharge, filed June 6, 1955. Inasmuch as the actual plating current utilized in said application and that utilized in the present application both comprise a series of separate 'pulses, it is believed that the conclusions indicated by the experimental work on pulsedplating currents arrivedat in said application entitled Method for Plating by Condenser Discharge are valid also for variations in peak voltages involving the method set forth in the present application. Accordingly, for reference purposes, the experimental work specifically utilized in connection with said companion application will be set forth hereinafter for the purpose above indicated: 1

Voltage Example 1 Using the standard copper solution as above described, a copper anode and a steel plate of the same character and size as above described, said plate was immersed in a said solution as a cathode and subjected to a current apsolution it was found that the plating thickness was the same as in Reference Example 1, the adherence following the same 30 day salt spray test as above mentioned was better than in Reference Example 1 and the grain structure indicated was more densely packed than in Reference Example -1. i

Y I Voltage Example 2 Using a cadmium bar as an anode and a steel plate of the same size as used in the above reference examples as a cathode, the electrodes were placed in a standard cadmium solution as, above described and plating current according to the method of the invention was applied at 160 volts peak, density of 300 amperes per square foot at the peak with an average voltage of about 6 volts and an average current of about 8 amperes per square foot. The plating was continued for 7% minutes. On removal of the plate from the solution it was found that the thickness of cadmium depositedwas 0.0003 inch and that the cadmium density was about the same as with the plate referred to in Reference Example 2.

Voltage Example 3 ing, was not as thick on the side which had faced the anode as it was in the case of Reference Example 3 but that it extended further around the pipe and was of more uniform thickness throughout its extent around the pipe than in the case of Reference Example 3.

'Voltage Example 4 Theprocedures of Voltage; Example 1 were repeated with 'volts, 300 volts and 600 volts respectively. The results v75 volts were acceptable, particularly the grain Upon examination structure was good but approximately twice the time was required to secure comparable thickness and plating as that required in Voltage Example 1. The results with 300 and 600 volts peak occurrence were unsatisfactory due to insufficiently dense and insufliciently inherent grain structure. The plating was, however, accomplished very quickly.

Voltage Example 5 Voltage Example 6 The procedures of Voltage Example 3 were repeated with 75 volts, 300 volts and. 600 volts respectively. The results with 75 volts were acceptable, particularly the grain structure was good but approximately twice the time was required to secure comparable thickness and plating as that required in Voltage Example 3. The results with 300 and 600 volts peak occurrence were unsatisfactory due to insutficiently dense and insufliciently inherent grain structure. The plating was, however, accomplished very quickly.

It will be observed that in the foregoing examples. the;

density and adhesion of the plating is at least as good as the density and adhesion of the plating in the reference.

examples, but that the time of plating in the examples is materially less than the time of plating in the reference examples. Inasmuch as it is clearly apparent from the above voltage examples that shortening the time of plating has a directly adverse efiect. on both the density and adhesion, and increasing the plating time improves the density and adhesion, it will follow that by extending the plating period utilized in the examples given above to a period equal to the periods utilized in the reference examples, the density will be much greater and the adhesion characteristics much better than in the corresponding reference' examples.

Thus, itappears that plating from an'anod'e onto a base may be carried out materially more quickly than is now customary and that the plating applied to the base is at least as firm and as densely packed and possesses at least as high a level of adhesiveness than is obtained by present methods. With copper plating, these qualities are better by the present method than with conventional methods. In any event, when the period of time during which the plating is carried out is made equal to the period, of time required for securing satisfactory results by conventional methods of the type above indicated in the reference examples, then the density and adhesion of the plating applied by the present method is materially better than the density and adhesion of the plating applied by such conventional methods. 7

The effectiveness of the pulses appears acceptable if they commence anywhere in the last half of their respective.

half-wave forms, although the practical limit is probably the last one-quarter of a half-wave. improves its effectiveness with its sharpness in both its rise and its fall, and where no separate cut-off is used its sharpness increases rapidly as the end of the half-wave is approached. Hence, as short a pulse will be used as is possible in a given case in view of the current requirements and the source available. In most cases the last 15 to 25 degrees will be found best, for the plating pulses, and the last 2 to 5 degrees will be found best for the strippingpulses.

It will also be understood in the light of the Actually, the pulse foregoing that a number of variations may be made in both the method and the circuits employed to carry out said method of the present invention and that the hereinafter appended claims are intended to include such variations excepting assaid claimsby their own terms expressly require otherwise.

I: claim:

1. In a process for electroplating a metal from the group consisting of copper and cadmium onto a base member the step comprising: supplying electrical energy to the plating electrodes in a series ofuni-directional pulses wherein each of said pulses constitutes a portion of successive half-waves of one polarity of an alternating current wave-form, each of said pulses beginning with respect to each half-wave after thehigh point thereof and ending at the first following point of zero potential.

2. In a method of electroplating a metal from the group consisting of copper and cadmium onto a base member from an electroplating electrolyte in which the metal to be plated is dissolved and in which the base member is immersed, said base member being one of a pair of spaced electrodes the other of which is composed of the metal to be plated and is immersed into said electrolyte, the steps comprising: supplying electrical energy to the plating electrodes ina series of uni-directional pulses wherein each of said pulses constitutes a portion of successive half-waves of one polarity of an alternating current wave-form, each of said pulses beginning with respect to each half-wave after the high point thereof and ending at the first followingpoint of zero potential.

3. In the process of claim 2 the step comprising interspersing between each of said pulses further pulses in opposite polarity and comprising less than the last half of the negative portions of the half wave form.

4. The process of claim 2 wherein each of said pulses constitutes the last 20 degrees of each positive half-wave.

5. The process of claim 2 wherein each of said pulses constitutes the last 30 degrees of each positive half wave.

6. The process, of claim 2 wherein each of said pulses constitutes about the last 20 degrees of each positive half wave and including the additional step of supplying a further series of uni-directional pulses intermediate said first named pulses and constituting a portion of successive half waves of polarity opposite to said first named pulses of said alternating current wave form, each, of said last named'pulses beginning with respect to each negative half wave after the maximum point thereof and ending at the first following point of zero potential.

7. In a method of' electroplating a metal selected from the group consisting of copper and cadmium onto a ferrous base member from an electroplating electrolyte in which the metal to be plated isdissolved and in which the base member is immersed, said base member being one of a pair of spaced electrodes the other of which is composed of the metal to be plated and is immersed into said electrolyte, the steps comprising: supplying electrical energy to the plating electrodes in a series of unidirectional pulses wherein each of said pulses, constitutes a portion of successive half-waves of one polarity of an alternating current wave-form, each of said pulses beginning with respect to each half-wave after the high point thereof and ending at the first following point of zero potential 8. The process of claim 6 wherein the second named group of pulses occupy about the last 5 degrees of said half-wave form of said opposite polarity.

References Cited in the file of this patent UNITED STATES PATENTS 1,566,265 Antisell Dec. 22, 1925 2,046,440 Adey July 7, 1936 2,451,341 Iernstedt Oct. 12, 1948 2,550,089 Schlesman Apr. 24, 1951 2,615,841 Thorp et al. Oct. 28, 1952 

1. IN A PROCESS FOR ELECTROPLATING A METAL FROM THE GROUP CONSISTING OF COPPER AND CADMIUM ONTO A BASE MEMBER THE STEP COMPRISING: SUPPLYING ELECTRICAL ENERGY TO THE PLATING ELECTRODES IN A SERIES OF UNI-DIRECTIONAL PULSES WHEREIN EACH OF SAID PULSES CONSTITUTES A PORTION OF SUCCESSIVE HALF-WAVES OF ONE POLARITY OF AN ALTERNATING CURRENT WAVE-FORM, EACH OF SAID PULSES BEGINNING WITH RESPECT TO EACH HALF-WAVE AFTER THE HIGH POINT THEREOF AND ENDING AT THE FIRST FOLLOWING POINT OF ZERO POTENTIAL. 